1. Technical Field
The present invention relates to a multi-beam antenna system and a method for controlling an output power thereof, especially to a multi-beam antenna system and a method for controlling an output power thereof that can control a strength of a beam by controlling an output power without recomposing a form of the beam.
2. Background Art
A beam-forming technology is currently used by many kinds of communication systems to avoid wasting of resources, and the beam-forming technology is realized by utilizing a multi-beam antenna system. Although multi-beams can be realized easily as an SFPB (Single Feed per Beam) system, in which each of feeds forms an individual single beam, its performances are not suitable enough for long distance communication due to a low gain. While a system having a multi-beam service coverage by having a plurality of feeds overlapped with one another can improve the directivity and gain of the beam greatly, it is difficult to recompose the beam in accordance with a change of situations. A phase array antenna is suitable to enforce the function of recomposing the beam. The phase array antenna for forming a multi-beam antenna uses a plurality of feeds for making a single beam, and a one feed may be involved in forming a plurality of beams due to overlapped beams. Making a beam with the phase array antenna requires an optimization process because a suitable amplitude and phase of a signal need to be excited to a feed. There are three methods of optimizing an excitation coefficient: optimizing both an amplitude and a phase (Amplitude & Phase Optimization: A&P), optimizing a phase only (Phase Optimization: PO), and optimizing an amplitude only (Amplitude Optimization: AO).
The latest phase array antenna system includes a beam-forming network (BFN hereinafter), a Multi-Port Amplifier (MPA hereinafter), and a feed array. The BFN, which is a beam creation portion creating a beam, optimizes an amplitude and phase of an excitation signal by including a variable attenuator and a variable phase shifter. The MPA controls an output power according to communication traffic of a service coverage formed by a plurality of means. The MPA is located between the BFN and the feed array, and includes an input matrix, a high power amplifier, and an output matrix. An output signal from the BFN is inputted to the input matrix of the MPA, and then the excited signal of which output is amplified by the high power amplifier in MPA is excited to a feed of the feed array. The MPA is published in “An adaptive multiple beam system concept” (S. Egami, M, Kawai, IEEE Journal on Selected Areas in Communication, Vol. SAC-5, No. 4, May 1987).
FIG. 1 shows an example of a conventional phase array antenna system.
The conventional phase array antenna system of FIG. 1 is the conventional phase array antenna system disclosed in U.S. Pat. No. 5,115,248 “Multi-beam antenna feed device” and includes a plurality of BFNs 100, an MPA 200, and a feed array 300. The BFN 100 of FIG. 1 includes a divider 101, a variable phase shifting portion 102, and a combiner 104. Moreover, the MPA 200 has an input matrix 202, a high power amplifier 203, and an output matrix 204.
The plurality of BFNs 100 of the phase array antenna system of FIG. 1 are provided in correspondence with the number of signals (B1˜BNb), each of which is outputted as a beam, and then each divider 101 of the plurality of BFNs 100 divides the corresponding input signals among the plurality of input signals (B1˜BNb) by the number (Ne) of the feeds, and then the combiner 104 receives and combines the signals divided by Ne number and phase-adjusted by the Ne number of the variable phase shifters, and transfers the signals to the MPA 200 as excited signals. Since the phase array antenna system of FIG. 1 has the variable phase shifting portion 102 but does not have a variable attenuator, it is possible that a multi-beam is formed with phase optimization (PO) only. The MPA 200 re-divides the excited signals inputted from the plurality of BFNs 100 in accordance with a signal amplification degree at the input matrix 202, and transfers them to corresponding high power amplifiers among the high power amplifiers 203. Then, the output matrix 204 receives the amplified excited signal inputted from each of the high power amplifiers 203 and inputs the amplified excited signal to a corresponding feed among a plurality of feeds 301, thereby exciting the corresponding feed.
The phase optimization (PO) can reduce the number of kinds of amplifiers and improve power usage efficiency because the output of the high power amplifier 203 does not need to be modified although an antenna gain is reduced. On the contrary, amplitude & phase optimization (A&P) takes advantage of optimizing of the antenna gain, but because of low power usage efficiency and high DC power consumption, the phase optimization (PO) is largely used to optimize an array antenna. Here, as many variable phase shifting portions 102 as the number of beams (Nb) multiplied by the number of total feeds (Ne) are required in order to form a perfectly recomposed beam for each beam. However, the phase array antenna system of FIG. 1 has a limitation in optimizing the antenna gain because it uses only the variable phase shifting portion 102 without the variable attenuator.
Moreover, it is difficult to realize and costly to produce the phase array antenna system of FIG. 1 because the phase array antenna system of FIG. 1 needs to have as many BFNs 100 as the number of beams (number of signals), a plurality of the variable phase shifting portions 102 are required despite much freedom in forming a beam as each BFN 100 divides and combines as many input signals as the number of feeds, and the number of ports for the divider 101 and combiner 104 are increased.
Meanwhile, a tradeoff between antenna gain optimization and power efficiency is described in “Flexible payload architecture for multi-beam shaped-beam coverage from a S-DMB Geostationary satellite system” (Piero G., Nicola G., and Piero A., Antenna and Propagation, EuCAP, November, 2006, pp. 1-6). The thesis shows the phase optimization (PO) has a minimum EOC (Edge of Coverage) gain degraded by 0.5˜1.8 dB, compared to the amplitude & phase optimization (A&P) by use of a reflector operating in S band (2 GHz˜4 GHz) and feed array. However, choosing the phase optimization (PO) is suggested because it is more efficient considering a saturated power of the high power amplifier 203 and DC power consumption even though the EOC gain is degraded. Thus, generally taken for a multi-beam antenna system controlling an output power of a satellite is the phase optimization (PO), which controls only a phase of an excited signal in order to make operational points of the high power amplifier 203 in the MPA200 identical.
FIG. 2 shows a beam pattern and an EOC gain in accordance with amplitude & phase optimization (A&P) and phase optimization (PO).
In FIG. 2, a reflector and a feed operating at Ka band (20˜30 GHz) are used, and (a) shows a case where amplitude & phase optimization (A&P) is applied and (b) shows a case where phase optimization (PO) is applied. Moreover, the dotted lines in (a) and (b) of FIG. 2 indicate a coverage where a communication service is required, and the solid lines indicate an outline of minimum EOC gain by optimization. As illustrated in FIG. 2, in the Ka band, beam patterns used in the phase optimization (PO) are degraded, compared to the amplitude & phase optimization (A&P), by radiating a beam to an unnecessary area, and the minimum gain was lowered by 0.5 dB˜1.4 dB. Although the gain for the Ka band is illustrated in FIG. 2, it is known that the amplitude & phase optimization (A&P) can improve the antenna minimum EOC gain by about 1 dB, compared to the phase optimization (PO), regardless of a frequency. However, as described above, the phase optimization (PO) is largely used even though the EOC gain is degraded by the problems of power usage efficiency and DC power consumption.